Adjust font texture cache based on device density.

The main purpose of this CL is reducing font cache size of
low-density device.

The memory usage for the small RGBA texture will be
Nexus 6P: 7,928,856 bytes (1408x1408)
Nexus 5X: 4,734,976 bytes (1088x1088)
These used to be 4,194,304 bytes

Test: manually checked
Bug: 64400885
Change-Id: Ied064a6d59909ad7fbeff74332973206436fbd34